An NMOS transistor can be turned on by applying to its gate terminal a voltage that is somewhat higher than a voltage applied to its source terminal. The gate-to-source voltage Vgs modulates the conductivity of the MOSFET's channel, which is formed between the drain and source terminals when Vgs exceeds a threshold voltage. Bootstrap circuits have been designed to provide MOSFET switches with a substantially constant conductivity by effectively inserting a voltage source (providing a bootstrap voltage) between the source and gate terminals of an NMOS transistor, when turning on the transistor. Such a bootstrap circuit may be used, for example, when the switch needs to operate with a relatively low supply voltage, or when it is important to preserve an analog waveform accurately from one side of a switch to the other.
Prior art FIG. 1 illustrates an example of a bootstrap circuit. An NMOS transistor N0 has a source terminal T1 connected to a variable input voltage, and it has a drain terminal T2 providing an output voltage. For turning on N0, switch S1 is closed to cause a gate terminal to be applied a voltage that is substantially the input voltage at the source terminal T1 raised by a bootstrap voltage, provided by a voltage source 12. Since the bootstrap voltage is higher than a threshold voltage for N0, N0 turns on, and a substantially constant Vgs voltage for N0 ensures that the conductivity between terminals T1 and T2 is substantially independent of the input voltage applied to terminal T1. To turn N0 off, switch S1 is turned off and switch S2 is turned on to discharge the gate terminal to ground.
By maintaining a substantially constant voltage between two device terminals, a bootstrapped semiconductor may operate reliably as a switch for voltage signals that may swing in a voltage range wider than a reliability voltage limit for the semiconductor device. A reliability voltage limit is a limit for the voltages that may be applied between two designated device terminals. By exceeding any one of a semiconductor device's reliability voltage limits, the device may become damaged or be subject to an accelerated deterioration of its electrical properties. A gate-oxide breakdown voltage, for example, is a reliability voltage limit for a MOSFET device. A junction breakdown voltage is another example of a reliability voltage limit.
The maximum excursion of a voltage signal that may be applied to a traditional transmission-gate MOSFET switch, an NMOS and a PMOS connected in parallel, without exceeding a gate-oxide breakdown voltage, is substantially the same as the gate-oxide breakdown voltage. What is needed is a simple, small, and efficient switch circuit that can handle relatively high input voltages without exceeding reliability voltage limits. Such a switch circuit may be used advantageously in a wide range of signal-processing and power-management applications. Further, a symmetrical bootstrapped switch circuit is needed for use in analog applications, such as switched-capacitor circuits, including analog-to-digital and digital-to-analog converters, to reduce or prevent even-order distortion. Further, a bootstrapped circuit is needed to facilitate operation at low supply voltages.